Electrolytic processes in restorative dentisrty /certified fixed orthodontic courses by Indian dental academy

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Electrolytic processes in restorative dentisrty /certified fixed orthodontic courses by Indian dental academy

  1. 1. ELECTROLYTIC PROCESSES IN RESTORATIVE DENTISTRY INDIAN DENTAL ACADEMYLeader in Continuing Dental Education www.indiandentalacademy.com www.indiandentalacademy.com
  2. 2. CONTENTS♦ INTRODUCTION♦ HISTORY♦ DEFINITIONS♦ ELECTROFORMING♦ ADVANTAGES♦ MODE OF DEPOSITION♦ FABRICATION OF INLAYS AND ONLAYS♦ APPLICATION OF CERAMIC♦ THE METAL CERAMIC BOND♦ FLAWS IN ELECTRODEPOSITION www.indiandentalacademy.com
  3. 3. ♦ ELECTROFORMING IMPRESSIONS♦ COPPER FORMED DIES♦ SILVER FORMED DIES♦ BONDING TO PORCELAIN USING ELECTRODEPOSITION♦ ELECTROLYTIC ETCHING♦ ELECTROLYTIC POLISHING♦ CONCLUSION♦ REFERENCES www.indiandentalacademy.com
  4. 4. INTRODUCTIONMaterials used for restorations to remain in themouth over the long term period must meetextraordinarily high requirements. They must beable to withstand a variety of diverse mechanical,chemical, thermal and osmotic changes withoutundergoing any esthetic or functional alteration.Electroforming or electrodeposition technologyprovides treatment characterized by a good longterm prognosis while meeting esthetic andbiological requirements. www.indiandentalacademy.com
  5. 5. HISTORY♦ The German scientist Jacobi laid the groundwork in 1837 for all subsequent industrial and dental electroforming.♦ Electroforming techniques have been used in dentistry for more than 50 years. In 1935, Daimano and Viverihofi fabricated the first electroformed dies from hydrocolloid impression materials.♦ Since 1950s , electroplating of mercaptan rubber were performed.♦ In 1962, Armstrong and Rogers produced gold copings with a thickness of 250 ųm. www.indiandentalacademy.com
  6. 6. ♦ The first ceramic veneered crowns were produced by Rogers in 1979.♦ In 1971, Weismann was awarded the patent for fabrication of dental prostheses using dies coated with silver lacquer.♦ In 1989, Weiland Edelmetal Company (Germany) introduced the first electroforming unit – the Auto Galvano Crown Device. www.indiandentalacademy.com
  7. 7. DEFINITIONS♦ ELECTROPLATING – This is the process in which the workpiece (cathode) is plated with a different metal (anode) while suspended in a bath containing a water base electrolyte solution. Also called electrodeposition♦ ELECTROFORMING –is a highly specialized process of metal part fabrication using electrodeposition in a plating bath over a base form or mandrel which is subsequently removed, thus the coating itself becomes the product. This differs from electroplating basically because the skin is much thicker and can exist as a self-supporting structure if the original matrix is removed. www.indiandentalacademy.com
  8. 8. ♦ ELECTOLYTIC ETCHING – The process by which the surface metal is electrochemically removed to create microscopic three dimensional relief for micro mechanical retention.♦ ELECTROPOLISHING –It is the controlled removal of a layer of surface metal using a combination of chemicals and electrical current.This procedure is essentially opposite to electroplating in which the metal part is the anode. Used for polishing of base metal alloys. www.indiandentalacademy.com
  9. 9. ELECTROFORMING♦ Electroforming is an alternative to cast metal technology with significant advantages.♦ It is an electroplating process where a thick metal layer is deposited onto a mandrel, or original to be replicated, and is then separated from it.♦ The part thus obtained is called an electroform. The main advantage of electroforming is that it is an atomic scale process assuring replication fidelity that is unmatched by any other technology. www.indiandentalacademy.com
  10. 10. ADVANTAGES OF ELECTROFORMED RESTORATIONS♦ HIGH BIOCOMPATIBILITY♦ PRECISION OF FIT♦ PULP PROTECTION♦ EASE OF LUTING♦ ESTHETICS♦ REASONABLE COST OF FABRICATION www.indiandentalacademy.com
  11. 11. ♦ BIOCOMPATIBILITY–a) Use of inert materials like ceramic and gold.b) Does not require biologically questionable unbound oxide layer for bonding as in metal ceramic restorations.c) Soldering is also avoided because solder contains non precious metals subject to corrosion.d) Does not contain non homogenous lattice structures resulting from heat, cold,inclusions etc.e) Minimal superficial flaws and impurities all of which reduce the corrosion and release of toxic by products. www.indiandentalacademy.com
  12. 12. ♦ PRECISION OF FIT- Based on the technical fabrication process, the precision of fit surpasses that of other ceramic restorations and also that of cast restorations made of base metal alloys or precious metals.♦ Hanning et al stated that there is less than 50 µm marginal gaps in the electroformed inlays .♦ Schafers reported marginal gap measurements made at 375 points in electroformed inlays and crowns. More than 80% of measurements were less than 20 ųm.♦ This is also attributed to burnishing or adapting the ductile gold edges of the restoration. www.indiandentalacademy.com
  13. 13. ♦ PULP PROTECTION – Due to their reduced space requirement, it is hard tissue sparing (pulp protecting) and is indicated for use in young patients with vital teeth.♦ The fine gold coping is 0.2 mm thick provides adequate space for ceramic with appropriate rigidity unlike cast metal framework of ceramo metal restorations which is 0.7mm thick.♦ All ceramic restorations, although esthetic, require the largest amount of hard tissue reduction during tooth preparation (1.5 – 2 mm). www.indiandentalacademy.com
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  15. 15. ♦ EASE OF LUTING – zinc phosphate cement is the material of choice for luting unlike use of technique sensitive resin materials for all ceramic restorations.♦ ESTHETICS – Major ceramic component and the gold color of the coping result in the esthetic appearance of the restorations.♦ RESONABLE COST OF FARICATION – The fabrication cost is less than that of the full cast crown. Also the gold used in the baths is recyclable depending upon the specific unit. It offers broad indications for use in inlays,onlays, crowns, fixed partial dentures, prostheses with implants and electroformed bases in complete denture prostheses. www.indiandentalacademy.com
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  17. 17. MODE OF DEPOSITION♦ A master cast of the tooth is prepared and coated with a die spacer to facilitate separation of the duplicating material.♦ The dies are duplicated and a conductive silver layer is applied to its surface.♦ The die is connected to a plating head which is the cathode.♦ The gold bath is the decisive element in deposition. Gold sulfite baths are commonly used in dentistry. www.indiandentalacademy.com
  18. 18. ♦ Earlier cyanide baths were used which can release toxic cyanide vapor if acids were added and hence were discontinued.♦ Gold in the bath exist in the form of a dissolved quaternary ammonium complex. The chemical formula for the ammonium-gold-sulfite complex is: (NH4)3[Au(SO3)2]. Amine is added in the bath is for stability.♦ This complex dissociates into a cathodic gold- amine complex and a sulfite ion. www.indiandentalacademy.com
  19. 19. ♦ The gold amine complex is destroyed on the cathode surface, leaving a thin deposit of fine gold, while the amine remains as a reduction product in the bath solution.♦ These reduction products are responsible for the limited life of the electrolytic baths. Extension of the bath’s life is possible by the addition of gold concentrate.♦ The gold remaining can be recovered from the floor of the container and can be recycled, sometimes up to 100%.♦ Electrodeposition of gold on a surface of 1cm² implies the deposit of 28 to 31 billion atoms of gold per second. www.indiandentalacademy.com
  20. 20. ♦ Such rapid formation of the fine gold layer does not permit formation of ideal crystal lattices. Thus, the hardness is increased approximately by a factor of 4 through laminar strain formation of crystal lattice.♦ This results in the galvanically deposited gold having a vickers hardness of 140 – 160.♦ The deposited fine gold with its extraordinarily fine lattice is recrystallized while being heated in the ceramic furnace which reduces the internal strains of the lattice structure. www.indiandentalacademy.com
  21. 21. ♦ The gold weight required for inlays and onlays varies between 0.2 and 0.9 g. The gold deposited reaches 99.90% purity.♦ The thickness of the gold copings or internal restoration is around 0.2 to 0.4 mm deposited for over 10 – 12 hours.♦ The melting point of the deposited gold, 1063ºC, and the relative high thermal coefficient of expansion, 15.2 x 10¯ 6/K in the temperature range of 25ºC to 500ºC and its firing stability makes it suitable for ceramic veneering. www.indiandentalacademy.com
  22. 22. FABRICATION OF INLAYS AND ONLAYS♦ The dies are duplicated with silicone and dies can be made with epoxy resin or die stone.♦ Special attention is required to ensure that undercut regions and transverse grooves are carefully blocked out. www.indiandentalacademy.com
  23. 23. ♦ Care has to be taken against excess blockout occlusally which may cause fracture of the onlay.♦ Conducting silver lacquer is applied precisely to the preparation margin.♦ To avoid deformation caused by ceramic shrinkage, layer thickness of the inner wall should be no less than 0.16 mm. www.indiandentalacademy.com
  24. 24. ♦ The lacquered die is then fitted for electroforming.♦ Various devices for electroforming are available over the years from the small single unit device to high capacity versions.♦ Capacity of the unit varies from 5- 10 l, and having a gold content of 100- 150 g. www.indiandentalacademy.com
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  26. 26. The most critical step in the electroforming process is the exact buildup and cutback. For this, rubber polishers of diverse sizes are used.♦ Beginning with the proximal surfaces, the electroformed margins are ground at an angle of 90 degrees down to the preparation margin with a soft rubber polishing wheel. www.indiandentalacademy.com
  27. 27. ♦ The gold margin is now thinned starting at the inside.♦ Occlusally, excess material is first removed in the direction of the preparation and the margins are reduced precisely in the direction of the cusp slope.♦ It is important that the electroformed margins have the exact anatomical shape approximally and occlusally to avoid later grinding with diamonds after ceramic firing which leads to shearing and widening of margins. www.indiandentalacademy.com
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  29. 29. APPLICATION OF CERAMIC♦ After the inlay is properly fitted on the working die, the margins are covered with wax to protect them during retentive sandblasting with 150 µm grain size material. www.indiandentalacademy.com
  30. 30. ♦ The wax is removed and the bonding agent is applied and firing is performed.♦ Next opaquing agent is applied which is a stiff paste that can be thinly feathered down to the margins. Additional shades of blue is applied at the margins and orange in the cavity and is fired again. www.indiandentalacademy.com
  31. 31. ♦ After firing the fit is checked and application of ceramic begins.♦ In the fissure region orange color tint is used.♦ Below the gold margin, blue tint is used and is covered with dentin ceramic starting at the fissure. www.indiandentalacademy.com
  32. 32. ♦ White incisor material is added moving in the direction of margins and is covered with transparent material.♦ Partial drying is done with a fleece.♦ The still slightly moist ceramic is then cut with a very thin,elastic blade in the regions of the main fissures down to the bottom. www.indiandentalacademy.com
  33. 33. ♦ The result is a tension free and homogenous firing in the direction of the margins.♦ The ceramic is then dried and the inlay is carefully lifted from the die and fired.♦ Thereafter ceramic is placed in the open cut regions and corrective firing is done. www.indiandentalacademy.com
  34. 34. ♦ It is shown that an inlay that fractures is commonly due to breaking open of cavity caused by insufficient closing of the gap.♦ The inlay is placed on the die, then in the articulator for finish- grinding of the occlusion and proximal contact points and is painted and glazed. www.indiandentalacademy.com
  35. 35. ♦ The inlay is finish- fired, it is placed back on the die and the margins are polished with a small brush and diamond paste and is cleaned with a steam jet.♦ Try-in is done and the fit and the color is checked & cemented. www.indiandentalacademy.com
  36. 36. ♦ Plastic veneers can also be used instead of ceramic for electroformed inlay. The difference is in the bonding agent (rocatec) to achieve a chemical bond with a synthetic material.♦ An all ceramic insert e.g empress ceramic material can also be combined with an electroformed “cavity liner”.♦ This technique is used when all ceramic inlay is needed but the preparation in mesial or distal region lies below enamel margin where adhesive bonding is not possible. www.indiandentalacademy.com
  37. 37. ♦ In this case, the electroforming technology can form a precise base for creation of an esthetic ceramic inlay which serves to fix the inlay in position. www.indiandentalacademy.com
  38. 38. THE METAL CERAMIC BOND♦ The theories of metal ceramic bonding assume that an optimal bond is achieved by means of an oxide film.♦ As pure gold by itself has no tendency to oxidize, a gold bonding agent containing ceramic particles provides an intermediate step.♦ The gold particles themselves consist of finer gold spheres in which ceramic particles are evenly distributed. www.indiandentalacademy.com
  39. 39. ♦ The first step is cleaning and roughening of the surface.♦ Sandblasting with particle size larger than 125µm and blasting pressure less than 2 bar is used.♦ Sandblasting gives the surface its typical matte appearance.♦ Right-angled blasting avoids the formation of gold lump and the danger of blistering the ceramic.♦ The gold bonding agent is applied to the clean metal with a brush, dried in a kiln and then fired. www.indiandentalacademy.com
  40. 40. ♦ During firing the gold particles fuse, flow together, and completely or partially incise the ceramic particles.♦ The individual ceramic particles are partially held by the fused gold particles and present their surface to the opaquer of the dental ceramic so that the ceramic can bond or mechanically interdigitate.♦ In the regions devoid of ceramic particles, the confluent gold particles of the bonding agent ensure a net like structure where the ceramic can bond mechanically. www.indiandentalacademy.com
  41. 41. ♦ Because of the small size of the particles, the firing temperature of the gold particles in the bonding agent is below the melting point of gold.♦ Thus a close diffusion with the gold frame, a gap free bond between the bonding agent and gold coping is formed. www.indiandentalacademy.com
  42. 42. MECHANICAL BONDING MECHANISMS♦ The principle behind this bonding mechanism is interdigitation between metal and ceramic at the marginal surface of contact, produced by roughening that surface.♦ To this end, the metal surface is roughened with sandblasting which cleans the surface through abrasion, improves wettability and alters the surface compositions through localized fusions.♦ But dental porcelain does not require a roughened area to bond to metal. www.indiandentalacademy.com
  43. 43. ♦ In fact porcelain will fuse to a well polished surface (Lacy et al 1977) but some surface roughness is effective in increasing bonding forces (Yamamoto et al 1985).♦ A basic principle of metal ceramic systems is that the thermal expansion of the metal should be greater than that of ceramic. www.indiandentalacademy.com
  44. 44. ♦ The difference in thermal expansion during cooling process produces tension that lead to the stabilization of the bond -compression bonding. www.indiandentalacademy.com
  45. 45. VAN DER WAALS FORCES♦ This refers to a bond by means of an electrostatic attraction between two atoms that approach each other within a space in which no chemical bond is effective.♦ Van der waals forces are achieved through dipole attraction.♦ They have little direct influence on the bonding strength but are important because of their ability to improve the surface wetting of a metal by the viscous ceramic during firing. www.indiandentalacademy.com
  46. 46. CHEMICAL BONDING MECHANISMS♦ Chemical bonding systems are the primary forms of bonding include atomic bonds, polar bonds and metallic bonds.♦ The ceramic materials form a crystal lattice consisting of positively and negatively charged ions, which are held together because of the mutual attraction of variously charged particles. www.indiandentalacademy.com
  47. 47. ♦ There exists a chemical bond between the ceramic and the oxides on the subsurface of metal which is the most significant mechanism♦ But in precious metals like gold, the oxide layer does not form and the placement of the bonding agent is the primary mechanism of bonding in electroformed restorations. www.indiandentalacademy.com
  48. 48. ALTERNATIVES AND NEWER TRENDS IN BONDING♦ The conventional method for ceramic veneering is by a bonding agent.♦ However, the bonding agent can concentrate in the corners of the restoration which is invariably thick. This may cause the piling up of ceramic particles suggesting the need for a ceramic bond without the bonding agent.♦ The fabrication of diamond burs with galvanically attached diamond particles suggest a similar way of anchoring the ceramic particles to the surface of the electroformed restoration. www.indiandentalacademy.com
  49. 49. ♦ In this, a dosing pump delivers a particulate suspension into the bath leading to direct electrodeposition of gold onto the surface.♦ During firing this surface layer reconstructs itself forming a tubular layer interspersed with cavities and the ceramic meshes with this layer securely. www.indiandentalacademy.com
  50. 50. FLAWS IN ELECTRODEPOSITION1) FAULTS IN TOOTH PREPARATIONi. If the transition from the restoration to the tooth is in antagonist contact regions, it will cause cracks in the ceramic.ii. A narrow isthmus design may cause breaking apart at this spot during fitting or cementation.iii. If the edges in the preparation are too angular, marginal cracks may develop. www.indiandentalacademy.com
  51. 51. 2) FAULTS IN WORKMANSHIPi. Improper application of bonding agent will cause bond failure.ii. In the absence of fixation of the particles of bonding agent, attaching forces find no resistance, will impede optimal wetting of the gold surface.iii. Insufficient compression of ceramic before firing causes centrally located point like defects in the ceramic inlays. www.indiandentalacademy.com
  52. 52. 3) FLAWS IN DEPOSITIONi. Flaws that interrupt the performance of the electroforming equipment are: Programming errors or incorrectly set values External interruption like power failures etc Improper placement of the dies Lack of conductive connection between contact wire and silver coating Moist plaster dies leading to bubble formation www.indiandentalacademy.com
  53. 53. ii. Contamination by foreign substances results in spongy surface.iii. Reddish brown precipitate is due to the formation of tiny spherical particles of gold instead of formation of a single solid layer. The reddish Color occurs as a result of light refraction of the spherical particles. www.indiandentalacademy.com
  54. 54. ♦ Low concentration of gold in the plating solution will cause irregular deposition in the form of globular structures or streaks and brownish black color www.indiandentalacademy.com
  55. 55. CLINICAL EXPERIENCE AND LONG TERM USE♦ The durability of electroformed restorations are comparable to that of other metal ceramic restorations.♦ Kaerschbaum et al stated that 5 year survival rate rate was 99%.♦ Leempoel et al found that the over a recall period of 10 years, the rate of successful restorations was 89.7% . www.indiandentalacademy.com
  56. 56. ELECTROFORMED DIES www.indiandentalacademy.com
  57. 57. ELECTROFORMING IMPRESSIONS♦ The metal dies that are produced from electroplated impression material have :i. High strength – wax patterns and gold castings can be easily burnished with little distortion to the die.ii. Adequate hardness- knoop hardness is 55- 80iii. Excellent abrasion resistanceiv. Detail reproduction of 4 µm or less is easily attainable. www.indiandentalacademy.com
  58. 58. ♦ Electroformed dies are usually made from polysulfide impression materials.♦ Other impression materials that can be electroplated are compound and silicones.♦ Hydrocolloid impression materials are difficult to electroplate and are not used.♦ The electric current may be supplied by dry cell storage batteries with a variable resistance and an ammeter. www.indiandentalacademy.com
  59. 59. ♦ A transformer and rectifier with a fixed resistance can be used where alternating current of 110V is converted to a direct low- voltage current suitable for plating.♦ A small container for the electrolyte with wire electrodes are used and a bar of pure copper or silver is the anode.♦ Acidic copper sulfate solution is used for copper plating.♦ Alkaline silver cyanide is the electrolyte in silver plating. www.indiandentalacademy.com
  60. 60. COPPER FORMED DIES♦ The popularity of copper plated dies began in 1930s and later silver plated dies became more popular.♦ Copper formed dies are made by electroplating compound or silicone impressions.♦ The cathode is the impression to be plated.♦ The anode is electrolytically pure copper and is immersed in the plating solution . The area of copper immersed is approximately equal to that of the impression to be plated.♦ The plating bath contains acid solution of copper sulfate. www.indiandentalacademy.com
  61. 61. ♦ Composition for solution for copper forming bath: Copper sulfate (crystals) - 200 gm Conc. Sulfuric acid - 30 ml Phenolsulfonic acid - 2 ml Distilled water - 1000 ml♦ Sulfuric acid increases the conductivity of the solution.♦ Phenolsulfonic acid serves to assist the penetration of copper ions into the deeper parts of the impression-THROWING POWER. www.indiandentalacademy.com
  62. 62. ♦ The surface of the impression is coated with a conductor of electricity – METALLIZING.♦ Impression compound is painted with a colloidal dispersion of graphite and is allowed to dry.♦ When the impression is a silicone rubber, finely divided copper powder is brushed on the surface to be plated.♦ About 15 mA is used to plate a single tooth impression.♦ Once a thin layer of copper is formed, the current is increased by 2-3 times the original. www.indiandentalacademy.com
  63. 63. ♦ Plating is allowed to proceed for 12 to 15 hrs.♦ During electrolysis, copper atoms give up 2 electrons (2e) and become Cu++ ions.♦ The Cu++ ion is attracted to the cathode where it gains 2e and is deposited as metallic copper.♦ The metallic copper of the anode regenerates the solution as the plating process occurs with the removal of copper as cathode.♦ ANODE: Cuº - 2e Cu++♦ CATHODE: Cu++ + 2e Cuº www.indiandentalacademy.com
  64. 64. ♦ The distance between the anode and the impression to be plated is important, the more greater the anode to impression distance, the more even is the quality of deposit.♦ About 6 inches is the suitable distance. Shorter distance causes excess copper to be deposited on the superficial surfaces leaving the deep areas inadequately plated.♦ The quality of deposit obtained with a freshly made solution is not as good as the one achieved when it has been in use for a short time. www.indiandentalacademy.com
  65. 65. ♦ Loss of water from evaporation should be replaced to maintain correct concentration of the electrolyte.♦ A sediment or sludge consisting of fine copper particles accumulates in the floor of the bath and the solution must be filtered.♦ When anodes containing a trace of phosphorus is used, the formation of sediment is considerably reduced. www.indiandentalacademy.com
  66. 66. SILVER FORMED DIES♦ With the advent of polysulfide impression materials, silver forming became popular.♦ The impression is made conductive by brushing the surface with silver which adheres to the rubber impressions.♦ Various metallizing agents are available like bronzing powder, aqueous suspensions of silver powder and powdered graphite.♦ The electroplating bath is a solution of alkaline silver cyanide and the anode is pure silver. www.indiandentalacademy.com
  67. 67. ♦ Composition of the silver bath is : Silver cyanide - 36 gm Potassium cyanide - 60 gm Potassium carbonate - 45 gm Distilled water - 1000 ml♦ Addition of acids must be avoided to the cyanide solution which causes release of poisonous hydrogen cyanide vapor.♦ The anode at least twice the area to be plated is used. www.indiandentalacademy.com
  68. 68. ♦ The greater the concentration of silver in the bath , faster the silver is deposited.♦ The polysulfide impression is cleaned thoroughly and dried and metallized.♦ An electrical contact is made with the metallized surface of the impression.♦ A direct current is applied for 10 hrs using 5 to 10 mA/cm² of cathode surface.♦ ANODE: Agº - e Ag+♦ CATHODE: Ag+ + e Agº www.indiandentalacademy.com
  69. 69. ♦ The impression is then filled with dental stone.♦ When the stone hardens, it is mechanically locked to the rough interior of the electroformed metal shell.♦ The impression material is then removed to provide a die with greater surface hardness and resistance to abrasion. www.indiandentalacademy.com
  70. 70. PROBLEMS IN SILVER FORMING♦ FAULTY CONDUCTION – The ammeter may show a current flowing, but the impression does not plate. This is caused by a short circuit through the electrolyte because of the exposure of the conducting wire to the solution.♦ EXHAUSTED SOLUTION – Plating is slow and the deposit is discolored. The solution must be discarded and replaced. www.indiandentalacademy.com
  71. 71. ♦ OVERCONCENTRATED SOLUTION – The ammeter reading drops rapidly to zero after the impression is placed in the bath. This is caused by over concentration of the solution is rectified by adding distilled water to it. An overconcentrated solution softens the impressions and causes discolorations of the cast.♦ SILVER ANODE TOO SMALL – An anode smaller than the impression leads to slow and irregular plating.♦ FRIABLE SILVER DEPOSIT – if the deposit is friable and granular, indicates current setting is too high. www.indiandentalacademy.com
  72. 72. BONDING TO PORCELAIN USING ELECTRODEPOSITION♦ Ceramic bonding to metal may require electrodeposition of metal coatings and heating to form suitable metal oxides.♦ First, a layer of pure gold is deposited onto the cast metal.♦ Next, a short “flashing” deposition of tin is done.♦ Alloys of cobalt – chromium, stainless steel, palladium – silver, gold alloys and titanium have all been successfully electroplated and tin coated to achieve satisfactory ceramic bonding. www.indiandentalacademy.com
  73. 73. ♦ ADVANTAGESA. Improves the wetting of porcelain onto the metal.B. Reduces the porosity at the metal – porcelain interface.C. The electrodeposited layer acts as a barrier between metal casting and porcelain to inhibit diffusion of atoms from metal to porcelain.D. The gold color of the oxide film enhances the vitality of the porcelain compared to the normal dark oxides. www.indiandentalacademy.com
  74. 74. ELECTROLYTIC ETCHING♦ The internal (tooth) surface of the cast restorations can be subjected to electrolytic etching for auxillary means of retention.♦ This procedure is done in base metal alloys leading to selective etching via dissolution of grain boundaries.♦ This leaves microscopic irregularities and the use of a composite resin capable of wetting these irregularities can increase the retention several fold. www.indiandentalacademy.com
  75. 75. ♦ The margins and the external surface of the restoration is are covered with sticky wax.♦ The restoration is used as an anode and stainless steel is the cathode.♦ A low voltage current is passed for 10 – 15 mins to create microscopic irregularities.♦ The tooth surface is also etched with phosphoric acid to create an irregular surface for the formation of resin tags. www.indiandentalacademy.com
  76. 76. ♦ Resin bonded fixed partial dentures are a more conservative alternative to conventional fixed partial denture.♦ MARYLAND BRIDGE is a type of resin bonded etched metal prostheses in which electrochemical pit corroding technique is used.♦ Earlier, retentive resin rivets extruding through the perforated framework were used (rochette bridge). www.indiandentalacademy.com
  77. 77. ♦ Livaditis & Thomson postulated that these rivets were exposed to increased stresses as well as abrasion & leakage diminished their longevity.♦ Thus they adapted electrolytic etching used by Dunn and Reisbick & Tanaka et al to produce pitting corrosion of metal for retaining acrylic resin facings on metal framework.♦ Livaditis et al used a 3.5% solution of nitric acid with a current of 250mA/cm² for 5 mins. This was followed by immersion in 18% hcl acid solution in an ultrasonic cleaner for 10 mins.♦ The metal framework is the anode and stainless steel cathode is used. www.indiandentalacademy.com
  78. 78. ♦ This will etch the internal surface of solid base metal.♦ The acid solution and technique were specific to the non beryllium nickel chromium alloy.♦ Subsequently, Thompson et al reported that 10% sulfuric acid at 300 mA/cm² and same cleaning procedures will produce similar results with beryllium containing nickel chromium alloy.♦ Sloan et al found that an electrochemically etched surface was 2.9 times more retentive than a perforated one. www.indiandentalacademy.com
  79. 79. ♦ Mc Laughlin et al reported a much faster technique for etching . This is by immersing them in a combined solution of sulfuric and hydrochloric acids placed in an ultrasonic cleaner for 99 seconds while electric current is passed through them.♦ Electrochemical etching is a technique sensitive procedure producing a gray matte surface.♦ Over etching produces an electropolished surface which is shiny and reflective. www.indiandentalacademy.com
  80. 80. ELECTROLYTIC POLISHING♦ It is a technique for polishing base metal alloys like cobalt- chromium etc which are very hard and difficult to polish.♦ After casting, sandblasting is done to remove any surface roughness/ green layer of oxide.♦ Then electropolishing is carried out. This procedure is essentially opposite to electroplating.♦ The rough metal surface is the anode in a bath of strong acid electrolyte.♦ A current is passed, causing the anode to ionize and lose a surface film of metal. www.indiandentalacademy.com
  81. 81. ♦ The first products of electrolysis will collect in the hollows of rough metal surface and prevent further attack in those areas.♦ The prominences of the metal surface will continue to be dissolved and in this way the contours of the metal surface are smoothed. www.indiandentalacademy.com
  82. 82. CONCLUSION♦ In view of the diversity of today’s alloys and newer emerging technologies, a superior and progressive treatment can be rendered to the patients.♦ Electrolytic processes presents with entirely new dimensions of quality to dental restorations, previously unattainable, hence a thorough knowledge of these procedures are necessary for every restorative clinician. www.indiandentalacademy.com
  83. 83. REFERENCES♦ Electroforming in restorative dentistry – Wirz – Hoffmann.♦ Restorative dental materials – Craig♦ Phillip’s science of dental materials – Anusavice♦ Fundamentals of fixed prosthodontics- Shillinburg♦ Introduction to metal ceramic technology – W. Patrick Naylor♦ Contemporary fixed prosthodontics – Rosensteil♦ Applied dental materials- John F Mc Cabe www.indiandentalacademy.com
  84. 84. ♦ Notes on dental materials – E C Combe♦ Tylman’s theory and practice of fixed prosthodontics - Malone♦ Journal prosthetic dentistry – 1980, 1982, 1984, 1994 www.indiandentalacademy.com
  85. 85. THANK YOUwww.indiandentalacademy.com

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